def main():
    N = int(input())
    A = tuple(int(input()) for _ in [0] * (N - 1))
    BC = tuple(tuple(map(int, input().split())) for _ in [0] * N)

    a = [0] + list(A) + [0]
    passenger = [0] * (N + 1)
    ans = 0
    for i, (b, c) in enumerate(BC):
        passenger[i+1] = passenger[i] + c - b
        ans += passenger[i] * a[i]
    print(ans)

main()